1. Field of the Invention
This invention relates to an optical switch and a method of operating such a switch and in particular to a linear coupler optical switch.
2. Related Art
When the waveguiding regions of optical waveguides are brought into close proximity, evanescent coupling of the fibre fields of an optical signal can occur leading to optical power transfer between the two waveguides. This is the basis of fused fibre couplers and polished couplers as described in Elect. Lett. 17, 243 (1981) "Fused Single Mode Access Couplers", CA Villarruel and RP Moeller and in Elect. Lett. 18, 929 (1982) "Single-mode Fibre Optic Directional Coupler" R Bergh, G Kotler and HJ Shaw, respectively. These are four-port linear couplers with one input port and one output port being provided by the ends of each waveguide. An optical signal coupled into one port of one waveguide will, if at an appropriate wavelength, initially propagate along that waveguide but will couple across to the other waveguide in a characteristic distance known as the coupling length which is a function of the wavelength of the optical signal.
The coupling length is also referred to as the half-beat length as it is half the length necessary for the optical signal to couple to the other waveguide and then couple back again, ie a single beat. A three half-beat coupler will couple an optical signal coupled to the input sort of one optical waveguide to the output port of the other waveguide; a two half-beat coupler will couple an optical signal coupled to the input port of one optical waveguide to the output port of the same waveguide.